Known as one conventional functional element by use of interaction between a surface acoustic wave and electrons in the semiconductor is a surface acoustic wave functional element of such a structure as to perform an interaction over the propagation path all in width of a surface acoustic wave. With respect to a surface acoustic wave amplifier as one example of surface acoustic wave functional element, for example, three structures have been proposed: direct type amplifier (FIG. 2), separate type amplifier (FIG. 3) and monolithic type amplifier (FIG. 4). The first direct type amplifier is of a structure that a piezoelectric semiconductor substrate 11 such as CdS or GaAs in possession of both piezoelectricity and semiconductivity is used to install thereon an input electrode 4, an output electrode 5 and an electrode 8 for applying a DC electric field to the piezoelectric semiconductor substrate 11, thereby amplifying a surface acoustic wave. However, no piezoelectric semiconductor in possession of both large piezoelectricity and large electron mobility has been found thus far. The second separate type amplifier is an amplifier of a structure with an input electrode 4 and an output electrode 5 provided on a piezoelectric substrate 1 of a large piezoelectricity and a semiconductor 12 of a large electron mobility disposed via a gap 13 as well. In this type of amplifiers, the amplification gain is largely affected by the flatness of the surface of the semiconductor and the piezoelectric substrate and by the magnitude of the gap. To obtain the amplification gain equal to practical use, it is required to make the gap as small as possible and keep it constant all over the operating region, so that there is an extreme difficulty in industrial production. On the other hand, the third monolithic type amplifier is an amplifier of a structure with an input electrode 4 and an output electrode 5 provided on a piezoelectric substrate 1 and a semiconductor 12 formed via a dielectric layer 14 rather than a gap 13 as well. According to 1970s studies by Yamanouchi and others (K. Yamanouchi et al., Proceedings of the IEEE, 75, p. 726 (1975)), an electron mobility for InSb of 1600 cm.sup.2 /Vs has been obtained in a structure with SiO coated onto the LiNbO.sub.3 substrate and a 50 nm thick InSb film deposited thereon and a gain of 40 dB has been obtained at a central frequency of 195 MHz under application of an extremely high DC voltage of 1100 V in a surface acoustic wave amplifier using this film. Since no good film quality of InSb was obtained, however, there has been a problem of too high driving voltage and too small amplification gain at a low voltage in consideration of applications to an actual portable apparatus.
Next, a surface acoustic wave convolver can be referred to as another application by use of interaction between a surface acoustic wave and electrons in a semiconductor. At present, surface acoustic wave convolvers arrest attention greatly as correlators for CDMA (Code Division Multiple Access) scheme of spread spectral communication. Since former times, digital LSI and analog LSI have been examined as CDMA correlators, but either of them was extremely large in power consumption, thus forming an extremely large barrier against applications to a handy device requiring a low power consumption. Thus, a surface acoustic wave convolver of zero consumed power in principle begins to be examined to practical use with advantages taken of low power consumption and no need for synchronism. In studies of a surface acoustic wave convolver, a convolution output of -59 dBm has been obtained at the system of InSb/LiNbO.sub.3, for example according to K. Yamanouchi, S. Mitsui and K. Shibayama, IEEE MTT-S Intern. Microwave Symp. Digest, p. 31 (1980).
To ensure applicabilities of a monolithic type amplifier to actual portable telephone or the like, however, it is required to obtain a better amplification gain at a practically low voltage of at least 9 V or lower and to implement it in a feasible process as well. In other words, a lower voltage than the former technique by two factors or more must be intended. Besides, as regards a surface acoustic wave convolver, a still greater efficiency must be attained.
In a former structure of surface acoustic wave functional elements, it has been required to lessen the thickness of a semiconductor film greatly in using such a semiconductor as InSb of a large electron mobility to match the electric impedance of a surface acoustic wave with that of the semiconductor. With a thin film thickness, however, the semiconductor film is poor in crystallinity and becomes smaller in electron mobility, so that no functional element better in characteristics has been obtained.
Besides, in convolvers, because of a small thickness of the semiconductor layer, there has been a problem for a method of taking out an output in a direction of thickness that no high efficiency is obtained and moreover a problem that the sheet resistance reduces, thus leading to a short circuit in the electric field of a surface acoustic wave has occurred for a greater thickness of the semiconductor layer. Furthermore, in a structure of forming a semiconductor layer above the propagation path, the loss of a surface acoustic wave has increased, thus causing a decrease in amplification gain and a lowering of efficiency.
Still further, no attention whatever has been paid to the presence of a buffer layer, the position of a grounded electrode and interaction in shape of a strip electrode.